Controlling the formation of perovskite crystals could help make solar power generation more affordable, reliable and efficient.
The performance of a promising solar cell material has been boosted by a global effort to understand and master its manufacture.
Researchers co-led by XUE Jingjing from the School of Materials Science and Engineering, Zhejiang University have used a technique called in-situ X-ray defraction to watch perovskite as it crystalises. They have used the findings to develop a better way to make perovskites, creating long-lived solar cells that convert sunlight to electricity with high efficiency.1
Perovskites are crystalline materials that absorb a broad range of wavelengths of visible light and transfer the energy to electrons to generate an electrical current.
Researchers hope to use perovskites for a faster, cheaper, and less energy-intensive way to make solar cells than the current method of using silicon. The most promising candidate perovskite is formamidinium lead iodide (FAPbI3). To make it, a solution containing formamidinium, lead and iodide ions is printed or coated in a thin film on a suitable surface. The solid FAPbI3 perovskite forms by fast crystallisation.
But that process can veer off course. Crystals of FAPbI3 come in two different forms. The desired light-absorbing state, known as the black phase. Or the non-light-absorbing yellow phase, which hampers solar cell performance.
Uncontrollable crystals
“The problem of fast and uncontrollable perovskite crystallisation is something I first encountered during my PhD studies, and I have been interested in unveiling the underlying mechanisms ever since,” Xue says.
“The crystallisation of perovskites is so fast, just seconds long, that it has been challenging to gain in-depth information about the process,” she says. “This has hampered more rational and targeted design of film processing methods for efficient FAPbI3 fabrication.”
Now, with researchers from the Lawrence Berkeley National Laboratory in California, US, Xue and her collaborators have watched perovskite crystallisation as it happens. The team used synchrotron X-ray diffraction analysis, which fires an intense beam of X-rays at the sample, then analyses the pattern of X-rays diffracting back, to track changes as the crystalline material forms.It showed that yellow phase FAPbI3 forms during the very earliest stage of crystalisation, called nucleation, as well as during crystal growth.
After revealing the importance of early intervention to prevent yellow phase crystal formation, the researchers then showed how an additive, pentanamidine hydrochloride (PAD), put the crystallisation process on track, minimising yellow phase formation.
PAD supported black phase crystal growth via an ‘oriented nucleation’ mechanism, Xue says. It bonds to and helps stabilise the black phase during crystallization, favouring its formation. “This enabled us to avoid the yellow phase of FAPbI3,” Xue says.
Efficient and stable
Adding PAD during perovskite manufacture also enhanced solar performance. Perovskites made with the additive reached a conversion efficiency of 25%, compared to 23.7% for those without PAD. That difference equates to 5% more electricity generated.
The solar cells also lasted longer. Following more than 1,000 hours of continuous illumination, the device using the new process retained 95% of its original solar conversion efficiency, much higher than the conventional device, which retained only 70%.
The team showed they could apply the ‘oriented nucleation’ technique to two different methods of FAPbI3 fabrication, suggesting the finding will have broad applicability to perovskite manufacture.
“Usually, strategies to promote the formation of black-phase FAPbI3 in one processing scenario fail when applied to others,” Xue says. “The universal mechanism we have discovered could facilitate the mass production of solar cell devices,” she says.
Reference:
1.Shi, P., Ding, Y., Ding, B. et al. Nature, 620, 323-327 (2023). https://doi.org/10.1038/s41586-023-06208-z